The present invention relates to a method for forming an operating voltage for an electronic device from a supply voltage by means for forming the operating voltage, in which method a minimum value and a maximum value are defined for the operating voltage. The invention relates also to an electronic device comprising means for forming an operating voltage, to form the operating voltage from a supply voltage, a minimum value and a maximum value being defined for the operating voltage.
A typical operating voltage value in integrated circuits, such as TTL circuits, has been 5 Vxc2x15%. This has led to the fact that several electronic devices have been designed to operate with the operating voltage of 5 V. In the design of integrated circuits, one important criterion is the so-called power density (W/mm2), which may, in turn, limit increasing the integration density. Reducing the operating voltage is one way of increasing the integration density whereby the power density value is retained in the allowed limits; consequently, circuit solutions have been developed which operate also at 3.3 V and even at 3.0 V. In portable electronic devices, a lower operating voltage also has the advantage that the power supply unit can be made smaller and lighter in weight, particularly if the power consumption does not require the use of high-capacity power supply.
Electronic devices have been developed, in which the allowed supply voltage range can be very wide, for example from 3.0 V to 5.5 V. One reason for such a wide range of supply voltage has been the compatibility of the electronic device to devices with different operating voltages. This is important e.g. in such electronic devices which are intended to be coupled to another device. One example that should be mentioned is expansion cards to be coupled to data processors. Such expansion cards can be used to change the properties of the data processors. It is possible to connect e.g. modem cards, network cards, audio cards, etc. to data processors, such as computers, wherein the computer can be used as a remote terminal or as a work station in the local area network of an office. By means of an audio card, the computer can be used to generate various audio signals and to perform digital signal processing.
Corresponding expansion card solutions, such as PCMCIA expansion cards, have also been developed for e.g. portable computers. The portable computer can be connected for example to a card-like wireless communication device with functions of a wireless communication device, such as mobile station functions. Thus, the portable computer can be used as a wireless modem, a mobile station, a wireless work station, etc. Typically, such an expansion card receives its operating voltage from the device to which the expansion card is connected. For the voltage supply of PCMCIA cards, several different operating voltages have been defined, e.g. 3.3 V and 5 V. Depending on the manufacturer or the type, the expansion card may require a certain voltage or it may operate with any of these voltages. Nevertheless, all the devices to which the expansion card can be connected do not necessarily have all said supply voltages available for the expansion card. Therefore, this possibility must be taken into account in the arrangements for the operating voltage of the expansion card, if the expansion card is intended to be compatible with as many devices as possible.
One such solution for forming the operating voltage of such an expansion card, or another electronic device, is that the expansion card is designed to function with the lowest allowed operating voltage value, and the operating voltage is set to this lowest value in the operating voltage supply block irrespective of the supply voltage, if it is within the allowed range. In the design, also certain ranges of variation (tolerances) are normally taken into account, wherein the expansion card will still function at an even smaller operating voltage than that designed. For example, if the expansion card is designed to operate at 3.3 V and the tolerance is 10%, the smallest operating voltage at which the expansion card will still function in a sufficiently reliable way, is ca. 3 V.
The above-mentioned reduction in the voltage will cause that some of the supply voltage will be converted to heat in the operating voltage supply block. Consequently, the greater the required reduction in the voltage, the greater typically the loss of power. Moreover, in case the supply voltage is close to the smallest allowed value, the power loss in the operating voltage supply block may cause that the operating voltage is below the allowed minimum. In a switched mode power supply (SMPS), the voltage reductions is brought about with a relatively good power transfer ratio, i.e. the power loss has been reduced. However, also this voltage supply solution has the problem that when the supply voltage is close to the allowed minimum value, the operating voltage supply block causes a voltage loss, which is typically some hundreds of millivolts, wherein the operating voltage to be supplied to the electronic device is below the minimum value. As a result, the electronic device does not necessarily function in a sufficiently reliable way in all situations.
It is an aim of the present invention to achieve a method in the voltage supply for an electronic device in such a way that a sufficient operating voltage can be secured for the electronic device also in the situation that the supply voltage is close to the determined minimum. The invention is based on the idea of forming two voltage supply circuits, of which the first one is used in a situation in which the difference between the supply voltage and the operating voltage is relatively large and, in a correrponding manner, with small differences between the supply voltage age the operating voltage, the second operating voltage supply circuit is used. The method according to the present invention is characterized in that in the method, the means for forming the operating voltage are provided with at least a first operating voltage supply block and a second operating voltage supply block, in which first operating voltage supply block the output voltage is limited smaller than said maximum value for the operating voltage; that at least a first limit value is determined; and that the supply voltage is compared with said first limit value, wherein if the supply voltage is greater than said first limit value, the first operating voltage supply block is activated to form the operating voltage from the supply voltage, and if the supply voltage is substantially smaller than said first limit value, the second operating voltage supply block is activated to form the operating voltage from the supply voltage. The electronic device according to the present invention is characterized in that the means for forming the operating voltage comprise at least a first operating voltage supply block and a second operating voltage supply block, in which first operating voltage supply block the output voltage is limited to be lower than said maximum value for the operating voltage; that at least a first limit value is defined in the electronic device; and that the electronic device also comprises comparing means for comparing the supply voltage and said first limit value, and control means for activating the first operating voltage supply block if the supply voltage is greater than said first limit value and for activating the second operating voltage supply block if the supply voltage is substantially lower than said first limit value.
Considerable advantages are achieved by using the present invention in comparison with solutions of prior art. Using the method of the invention, the operation of the electronic device can be secured on a wide range of supply voltage and it is still possible to maintain a good power transfer ratio also at higher supply voltages.
In the following, the invention will be described in more detail with reference to the appended drawings, in which